Recently, the need for management of product-specific information, management of tracking information, and the like has been increased and there has been a growing trend toward introduction of contactless tags. Among others, applications of a radio frequency identification (RFID) system using a tag and a reader have actively been studied. Such applications include one in which by conducting wireless communications in which a query signal is broadcast to a product to which a tag is attached, an object to which a tag is attached, or a human body to which a tag is attached and then after obtaining a response from such a tag, stock control of the product, observation of the moving object, or observation of behavior of the human is conducted.
Typical RFID systems use electromagnetic waves. On the other hand, there have also been demands to apply an RFID system to environments in which RFID systems cannot operate, such as an environment in which electromagnetic radiation is present, a situation in which the tag is embedded in water or metal, or an environment in which the tag is affected by electromagnetic shielding caused by metal. Moreover, there is a report that some types of electromagnetic waves may negatively affect the health of human bodies, so concerns about safety of electromagnetic waves have not completely been dispelled.
For a tag using electromagnetic waves, power to drive the tag, such as absorbing energy of a query signal to activate a chip inside the tag must be supplied by electromagnetic induction or via microwaves from the reader. Therefore, disposition of an internal circuit for power supply must be considered, thereby requiring a minimum of one complex system. Thus, under a harsh environment in which the tag is subjected to physical or chemical stress, such as that caused by staining of the surface, peeling, corrosion, or wear, there is no guarantee that reliability of operation of the tag will be maintained.
Ultrasonic tags, which use ultrasonic waves instead of electromagnetic waves, have been considered. These tags are active tags, a type of tag in which an ultrasonic transmitter is disposed for transmitting ultrasonic waves in the vicinity of the ultrasonic tag. In these ultrasonic tags, the communication distance between the tag and reader can be made longer to a degree that ultrasonic waves transmitted from the tag can reach the reader. These tags have been applied to technologies, such as one that calculates the distances from one tag to a plurality of readers by using the differences in the time taken for ultrasonic waves transmitted from the tag to reach the respective readers, and by identifying the three-dimensional location of the tag by three-point measurement based on these distances.
However, with regard to active tags, the use of a complex system, such as an energy supply system, is unavoidable due to the need to transmit ultrasonic waves near the active tags. Such a complex system will be costly in terms of battery replacement or charging.
On the other hand, passive tags have a relative advantage in that while they make some compromise in terms of communication distance, they require no battery and can easily be downsized. However, as for passive RFID tags using electromagnetic waves, complexity is unavoidable, such as including an IC chip and antenna.